Humans and chimps separated from a common ancestor six million years ago, while gorillas split off from that common ancestor four million years before that, confirms a new study, published in Nature.

Sixty scientists worked over five years to sequence the genome of a single female lowland gorilla, the last of the great apes to have its DNA mapped. According to lead author Aylwyn Scally, of the Trust Genome Campus of the Wellcome Trust Sanger Institute in England, even with that evolutionary distance, humans and gorillas have a lot more in common, genetically, than previously thought.

“The passage of ancestry across the three genomes changes from position to position," Scally says. "Although most of the human genome is indeed closer to chimpanzee on average, there’s a sizable minority, 15 percent is in fact closer to gorilla. And another 15 percent is where chimpanzee and gorilla are closest.”

Ninety-eight percent of human and gorilla genes are identical; humans and chimps share 99 percent of their DNA. Co-author Chris Tyler-Smith, also with the Wellcome Trust Sanger Institute, says it’s those relatively few genes that differ between the species that are of special interest.

Genes tell a story

For example, the study finds that a single gorilla gene associated with enhanced production of keratin - a protein that toughens the apes’ fingernails, skin and especially their knuckle pads - is absent from the human genome.

A group of genes, associated with hearing, tells another story.

“It’s been known for some time that hearing genes in humans have shown accelerated evolution," Tyler-Smith says, "but what we could see by sequencing the gorilla genome was that this acceleration goes back millions of years. So the implication of that is that this is not because of human language ability, it must be for some broader role that these play.”

The gorilla genome sequencing also identified several genes that cause disease in humans, but not in gorillas. One gene leads to a form of human dementia, a second is associated with heart failure in people.

“If we could understand more about why those variants are so harmful in humans, but not in gorillas, that would have important or useful medical implications,” says Tyler-Smith, who intends to explore the ancestral family tree further, to learn what happened as humans and apes evolved on their separate paths.

He says the gorilla sequence is a template that will help to explain many of those evolutionary mysteries.